Small Cell Enhancement (SCE), which is the focus for Rel-12 standardization, relates to many aspects, such as NCT of the physical layer. NCT is a key supporting technique of a physical layer of the SCE, and is first proposed in carrier aggregation enhancement of Rel-11. Therefore, the initially discussed scene of NCT is SCC, which serves User Equipment (UE) by way of carrier aggregation. WI of NCT in Rel-12 is approved in the RAN #57 meeting, and WID is updated in the RAN #58 meeting. The standardization work of NCT mainly includes two stages as followed.
Some characteristics of NCT are defined in RP-122028, mainly including: the NCT design has reduced traditional control signaling and common reference signals transmitted on carriers, thereby reducing the interference and transmission load of the control channel and improving the throughput capacity and the system frequency band utilization efficiency of users. The above characteristic of NCT can be better support edge users in homogeneous networks and cell range expansion areas of heterogeneous networks. Meanwhile, NCT can support new scenes, for example, NCT can allow a BS (base station) to switch off the current carriers when there is no data transmission, which further reduces the network power consumption and improves the energy efficiency. In standardization, NCT is classified into non-standalone NCT (referred to as NS-NCT) and standalone NCT (referred to as S-NCT). NS-NCT means that when a frequency resource block does not support independent operations of NCT, for example, in an asymmetrical FDD frequency spectrum scene, NCT can be used only after aggregation with a traditional LTE carrier. NS-NCT can be further classified into synchronized and unsynchronized carriers, wherein synchronized NCT does not need to transmit synchronizing signals and performs time-frequency domain synchronization by way of an aggregated traditional LTE carrier, while unsynchronized NCT needs to transmit synchronizing signals for synchronization.
Stage 1:
Standardization is performed to the scenes of NS-NCT. NCT coexists with backward compatible carriers through carrier aggregation. The above scene can be further classified into two different scenes of synchronized and unsynchronized carriers.
At the same time, this stage also includes studies on S-NCT, and assesses its main application scenes and potential advantages to determine whether it is necessary to study S-NCT scenes.
Stage 2:
Depending on the assessment results in Stage 1, if it needs to further study S-NCT, standardization needs to be performed for S-NCT scenes with reference to the SI study results of SCE and the determined optimization principles.
Currently, NS-NCT is the main scene discussed for standardization. The definition of NS-NCT has been approved in the RAN #57 meeting, i.e., when a target frequency resource block does not support independent operations of NCT, for example, in an asymmetrical FDD frequency spectrum scene, NCT can be used only after aggregation with a traditional LTE carrier.
In addition, consensus has been reached on the motivation of introducing NCT to carrier aggregation scenes in the RAN1 #66bis meeting, mainly including the following three items:                (1) to improve the frequency spectrum efficiency;        (2) to support deployment of heterogeneous networks; and        (3) to facilitate energy-saving.        
To realize the above three objectives, when designing NCT, some common control channels/signals, such as CRSs, should be removed as much as possible. However, CRSs are crucially important for some mechanisms, such as time-frequency synchronization of UE, RRM measurement and cell handover etc., so how to realize these mechanisms in the NCT scene is a major task for standardization. Main functions of CRSs are listed as below in the LTE Rel-8/9 version:                (1) to demodulate downlink data (TMs1-6), control channels and PBCH channels;        (2) to calculate CSI feedback (TMs1-8);        (3) to perform time-frequency domain synchronization of UE;        (4) to perform mobility measurement (RSRP/RSRQ) under RRC-IDLE and RRC-CONNECTED states; and        (5) to perform RLM measurement under the RRC-CONNECTED state.        
A non-codebook based pre-coded transmission mode TM9 is introduced in the LTE Rel-11 version. TM9 supports 8-layer transmission at a maximum capacity, increasing the transmission efficiency. TM9 performs data demodulation using demodulation reference signals (DM-RS); for the CSI feedback, estimates channel conditions using channel status information reference signal CSI-RSs to ensure the feedback accuracy, and estimates interference conditions using CRSs.
Based on the current discussion results, usable reference signals in NCT include the following types:
(1) PSS/SSS
Primary synchronization signals (PSSs) and secondary synchronization signals (SSSs) are mainly used to perform initial symbol synchronization and frame synchronization. For synchronized carrier scenes of NCT, since the synchronization information of a cell is obtained through a traditional carrier, PSSs/SSSs may be removed in NCT to further improve the resource utilization efficiency of NCT. However, some proposals show that the gains obtained through PSS/SSS removal are not obvious, greater influence will be caused to standardization and the complexity of UE will be increased. Therefore, currently there is no consensus on the removal of PSSs/SSSs in synchronized carrier scenes of NCT, and further discussion is needed still.
(2) DM-RS
Different from cell-specific CRSs, DM-RSs are UE-specific reference signals, transmitted in certain PRBs and used for demodulation of UE data channels. The DM-RSs of different UE may occupy the same RE distinguished by CDM. In addition, resource allocation for DM-RSs is finished before precoding, so DM-RSs include precoding gains. There is a problem of Collision between DM-RSs and PSSs/SSSs in NCT. According to current discussions of the 3GPP, PSS/SSS shifting and DM-RS puncturing are mainly considered to improve the performance of physical downlink shared channels (PDSCHs), to facilitate demodulation of PDSCHs/ePDCCHs (enhanced physical downlink control channels) and to avoid resource collision. In this way, support for future standardization evolution may help to be obtained.
(3) CSI-RS (Channel Status Information Reference Signal)
As DM-RSs in the R10 version, CSI-RSs are introduced to support 8-antenna configurations in LTE-A, to estimate channels conditions of PDSCHs and to realize beamforming. CSI-RSs are distributed with even intervals in the frequency domain, but are sparsely distributed in the time domain. Similarly, CSI-RSs occupying the same RE are distinguished by CDM. In addition, CSI-RSs are UE-specific reference signals and configured by the BS before use.
(4) Reduced CRS
Since there is no transmission of CRSs and ePDSCCH of NCT are demodulated based DM-RSs, the transmission mode of NCT does not support TMs1-8. Therefore, to replace CRS in NCT, problems to be solved include time-frequency domain synchronization, radio resource management (RRM) measurement and interference measurement under the TM9 mode. To solve the above problems (including synchronization and RRM measurement), the current discussion result is to increase Reduced CRSs (Reduced Cell-specific reference signals). Reduced CRSs still base on CRSs, use port0 ports and sequences in Rel-8, and are transmitted once by every 5 ms. Reduced CRSs are also called Traditional CRS (TRS), extended Synchronization Signal (eSS) etc.
Reduced CRS solutions are still under RAN4 discussion, since simulations find that performance loss is present in the scenes of relatively small carrier bandwidths. Therefore, if the conclusion of RAN4 is to increase the reference signal density, RAN1 needs to re-design Reduced CRS.
Much content is yet to not be determined for Reduced CRS. For example, whether subframe offset needs to be introduced to the position of the subframe of Reduced CRS? Obviously, introduction of subframe offset may potentially alleviate interference problems, but it will increase the complexity. Meanwhile, different companies disagree on whether cell-specific frequency offset should be maintained for Reduced CRS. In all, the specific content of Reduced CRS needs improvement.
A carrier aggregation mechanism is introduced to the LTE Rel-10 version to meet the requirement that the transmission bandwidth should reach 100 MHz in IMT-A. The carrier aggregation mechanism is mainly realized by RRM measurement. For carrier aggregation, the purpose of RRM measurement is not only to perform mobility management for UE but to realize activation and deactivation of component carriers.
RRM considers QoS parameters (QCI/GBR/AMBR) in a comprehensive way, including the prior conditions such as configuration of wireless load, the terminal reception capability and the carrier load condition, and configures one carrier set for each UE. Then, the UE measures the cells in its carrier set based on multiple measurement events defined by the standards, and reports the measurement result to the network side, which performs activation and deactivation to SCC based on the measurement result. Since UE may be configured with multiple component carriers (referred to as CCs), the UE must keep communication with one PCell and at most four SCells. The UE no longer performs cell measurement for handover, but selects the most suitable cell or cells to provide services based on the current radio environment. The UE may measure multiple cells using different measurement events. For A3 and A5 events, the reference cell is PCell which is providing services, and the measurement object may be any frequency or the SCell which is providing services; and A6 events only provide handover measurement among SCells of the same frequency. In the UE's carrier set, measurement of an activated cell should be consistent with the process defined in Rel-8, whose measurement interval is UE-specific, while the measurement interval of a non-activated SCell is configured by RRC signaling.
At the same time, activation/deactivation of component carriers (CCs) may be controlled by the network side. Now, the network issues a UE activation/deactivation MAC control unit to activate/deactivate SCCs, but the MAC layer only reports random access failure and retransmission failure problems of PCell to a higher level. Reporting of the channel quality indicator (CQI) is directed for an activated SCell only, and the radio link condition of a non-activated SCell cannot be provided. However, RRM measurement can be performed to activated or non-activated downlink secondary component carriers (DL SCCs). The RRM measurement result can reflect the current radio link quality of a DL SCC, and help the network side to decide if the corresponding SCell is suitable for providing services for the UE.
In an LTE/LTE-A system, a radio link management (RLM) mechanism is mainly used to monitor the radio link of a primary component carrier PCC to determine if the radio link status is normal, ensuring the reliability of the radio communication system. In the activation/deactivation of DL SCCs in Rel-10/11, the RLM mechanism is not applied due to the following reasons:
(1) the BS is capable of detecting whether the DL SCC radio link quality deteriorates based on CQI reporting (for activated DL SCCs) and the current RRM measurement reporting (for activated or deactivated DL SCCs) mechanisms;
(2) the RRM reporting mechanism (such as A2 event) is capable of reporting DL SCCs with deteriorated link quality; compared with CQI measurement, filtering of RRM measurement results has been performed at the UE side, and deactivation of DL SCCs can be performed so long as the network side configures RRM measurement for the UE;(3) after radio link failure (RLF) occurs to DL SCCs, the UE cannot automatically deactivate the corresponding SCCs so that the carrier sets at the eNodeB side and the UE side do not match; and(4) using the RLM mechanism for SCC activation control will increase the complexity of UE.
In Rel-10/11, the activation/deactivation of SCell is controlled by the eNodeB. Specifically, a traditional RRM measurement-based SCC activation/deactivation process is as below:
1. detecting the presence of a CC (acquiring the physical Cell ID through PSS/SSS);
2. acquiring the master information block (MIB) information (including the bandwidth, the PHICH configuration and the system frame number etc.);
3. measuring the signal quality of the CC (RSRP/RSRQ measurement based on CRSs);
4. performing measurement and reporting based on defined measurement and reporting events (such as A6 events and the aforementioned instantaneous RRM measurement); and
5. deciding by the BS whether to activate/deactivate the CC by the UE based on the measurement and reporting result.
At the same time, the network side may configure a timer for the UE side. When the UE does not receive data and PDCCH messages, SCCs may be deactivated automatically. The steps are as below:
1. the UE keeps one sCellDeactivationTimer timer for each SCell;
2. before the timer times out, the UE does not receive any data and PDCCH message; and
3. when the timer times out, the UE deactivates the corresponding SCell automatically.
However, in NCT scenes, PSS/SSS and physical broadcasting channels (PBCHs) may be removed, which will substantially affect the current carrier aggregation mechanisms. For example, once the PSS/SSS and PBCHs are removed, detecting CCs and acquiring the MIB of cells will become difficult. For synchronized NCT, the presence of NCT carriers and system information (such as PCI, SFN and bandwidth) can be indicated by traditional carriers. In addition, system bandwidth information may not be crucially important for RRM measurement (since the UE may only measure several RBs at the central frequency). In addition, since NCT is used as SCC only, the configuration information of physical hybrid-ARQ indicator channels (PHICHs) is unnecessary.
As shown by the above description, in NCT scenes, the configuration of reference signals is changed substantially, so that traditional SCC activation control solutions are not suitable for NCT scenes. Currently, there is no effective solution on how to perform SCC activation control in NCT scenes.